Spring : Real Life Based Introduction to Dependency Injection

Scenario 1 You work in an organization where you and your colleagues tend to travel a lot. Generally you travel by air and every time you need to catch a flight, you arrange for a pickup by a cab. You are aware of the airline agency who does the flight bookings, and the cab agency which arranges for the cab to drop you off at the airport. You know the phone numbers of the agencies, you are aware of the typical conversational activities to conduct the necessary bookings.
Thus your typical travel planning routine might look like the following :

Decide the destination, and desired arrival date and time

Call up the airline agency and convey the necessary information to obtain a flight booking.

Call up the cab agency, request for a cab to be able to catch a particular flight from say your residence (the cab agency in turn might need to communicate with the airline agency to obtain the flight departure schedule, the airport, compute the distance between your residence and the airport and compute the appropriate time at which to have the cab reach your residence)

Pickup the tickets, catch the cab and be on your way

Now if your company suddenly changed the preferred agencies and their contact mechanisms, you would be subject to the following relearning scenarios

The new agencies, and their new contact mechanisms (say the new agencies offer internet based services and the way to do the bookings is over the internet instead of over the phone)

The typical conversational sequence through which the necessary bookings get done (Data instead of voice).

Its not just you, but probably many of your colleagues would need to adjust themselves to the new scenario. This could lead to a substantial amount of time getting spent in the readjustment process.Scenario 2 Now lets say the protocol is a little bit different. You have an administration department. Whenever you needed to travel an administration department interactive telephony system simply calls you up (which in turn is hooked up to the agencies). Over the phone you simply state the destination, desired arrival date and time by responding to a programmed set of questions. The flight reservations are made for you, the cab gets scheduled for the appropriate time, and the tickets get delivered to you.
Now if the preferred agencies were changed, the administration department would become aware of a change, would perhaps readjust its workflow to be able to communicate with the agencies. The interactive telephony system could be reprogrammed to communicate with the agencies over the internet. However you and your colleagues would have no relearning required. You still continue to follow exactly the same protocol as earlier (since the administration department did all the necessary adaptation in a manner that you do not need to do anything differently).Dependency Injection ? In both the scenarios, you are the client and you are dependent upon the services provided by the agencies. However Scenario 2 has a few differences.

You don’t need to know the contact numbers / contact points of the agencies – the administration department calls you when necessary.

You don’t need to know the exact conversational sequence by which they conduct their activities (Voice / Data etc.) (though you are aware of a particular standardized conversational sequence with the administration department)

The services you are dependent upon are provided to you in a manner that you do not need to readjust should the service providers change.

Thats dependency injection in “real life”. This may not seem like a lot since you imagine a cost to yourself as a single person – but if you imagine a large organization the savings are likely to be substantial.

Implemented Scenario

The attached source pretty much implements the scenario described earlier. The general flow (starting from the main method in each “*Harness” classes) is as follows :

The Harness class is instantiated and the runHarness() method called.

The runHarness() method (in AbstractHarness) first invokes the configure() method. The configure() method typically initializes the DI container and registers the implementation classes.

The runHarness() method subsequently invokes the getPlanner() method. This typically invokes the lookup method on the underlying DI container. What is not immediately apparent, is that the lookup actually triggers the DI container to instantiate the implementation classes for AirlineAgency, CabAgency and TripPlanner interfaces, and resolve the dependencies between them.

AirlineAgency and CabAgency are interfaces for the respective services. CabAgency requires a reference to the AirlineAgency to be able to fulfill its expectations successfully.

TripPlanner is the interface for a service which can effectively plan out the trip in conjunction with the AirlineAgency and CabAgency. TripPlanner requires a reference to the AirlineAgency and CabAgency.

The UML diagram for the source is shown below :

Dependency diagram

The salient differences to be observed across different frameworks are

The configure() and getPlanner() method in each corresponding harness.

How the interface / implementor relationships are described (sometimes in java and sometimes in xml files)

How the dependencies between different services are declared (sometimes in xml, sometimes in the constructor and sometimes using a setter method)

Spring Framework

XML Declaration

The implementations that need to be instantiated are declared in the XML. Dependencies between services are also declared as “property” element. Spring framework will use this information to invoke the corresponding “setter” method.

<bean id="AirlineAgency" singleton="true"/>

Container Initialization

The container initialization typically requires a reference to the xml file and an instantiation of the BeanFactory.

References to the services are retrieved based on the ‘id’ specified in the xml (not the interface class). Again all the services are implicitly instantiated in the appropriate order and the setters called to resolve their dependencies.